In the past, techniques of leveling sound volumes (loudness) between different audio sources (musical pieces in the case of music) and improving comfort and convenience of a listener (for example, eliminating the inconvenience or the like of adjusting a volume for each musical piece) have been developed and put to practical use (for example, see Patent Literatures 1 to 3).
For example, in a known related art, an effective value of an entire musical piece is used to calculate a sound volume correction value. Further, in this case, a prefilter is used to improve a correlation between an auditory sensation and an effective value. As a characteristic of the prefilter, for example, a K characteristic or the like is often used.